This invention relates to hand held electrically powered hammers, and in particular to demolition hammers.
Such hammers generally comprise a housing within which is located an electric motor and a gear arrangement for converting the rotary drive of the motor to a reciprocating drive to drive a piston within a hollow spindle, which spindle is located within the hammer housing. The spindle may be formed from a single part or from more than one part, for example from a rearward hollow cylinder, within which a piston and ram reciprocate and a forward cylindrical tool holder body, within which a tool or bit may be releasably mounted. A ram is located in front of the piston within the spindle so as, in normal operating conditions, to form a closed air cushion within the spindle between the piston and the ram. The reciprocation of the piston reciprocatingly drives the ram via the air cushion. A beatpiece is generally located within the spindle and transmits repeated impacts that it receives from the ram to a tool or bit releaseably mounted for limited reciprocation in front of the beatpiece in a tool holder portion of the spindle. The impacts on the tool or bit are transmitted to a workpiece against which the tool or bit is pressed in order to break up or make a bore in the workpiece.
Some hammers may also be employed in combination impact and drilling mode in which the spindle, and hence the bit inserted therein, will be caused to rotate at the same time as the bit is struck by the beatpiece. The present invention is also applicable to such rotary hammers.
One problem with such hammers is that the reciprocating parts and repeated impacts between the parts cause large vibrations to be transmitted via the handles of the hammer to the user. This is uncomfortable for the user, particularly over prolonged periods of use and can contravene safety standards.
This problem has been solved in the past by forming a vibration damping linkage between the handles of the hammer and the main housing of the hammer. However, the linkages have to be rigid enough for the handles to guide the hammer while also providing damping. Also, the user of the hammer tensions the linkage when the hammer is urged against a workpiece and this changes the damping effect of the linkage. This means that such linkages tend to be relatively complex.
This problem has also been solved for a pneumatic hammer, for example as disclosed in DE815,179 by mounting masses on opposing sides of the spindle, with each mass mounted between two springs so that each of the masses can oscillate parallel to the axis of the spindle due to the forces from the two springs. The masses oscillate in phase and in the same direction as the ram and are arranged to oscillate as near to resonance as possible. However, this gives rise to the problem of synchronising movement of the masses. If the masses are not exactly synchronised then a torque at right angles to the direction of mass vibration is generated which is transmitted to the user of the hammer via the hammer housing. This problem has been addressed in DE31 22 979 which describes an electrically powered hammer to which a dampening housing is attached. The dampening housing comprises two moveable masses each connected to a compression spring. The channels in which the masses are located are interconnected so that generation of an over pressure in one channel results in a corresponding over pressure in the other channel in order to synchronise movement of the masses. However, the arrangement disclosed in DE31 22 979 is relatively complex and takes up a lot of space.
The problem of synchronising masses can also be overcome for a pneumatic hammer by using a single mass as described in DE24 03 074 in which there is described a hammer housing which is enclosed by a handle housing. Around the hammer housing is located a cylindrical mass which is able to reciprocate along the hammer housing on the end of a coil spring. Optimum vibration reduction is achieved if the spring constant of the coil spring is adapted to the beat frequency of the hammer.
A second problem is that the reciprocating parts and repeated impacts cause heat generation within the hammer and some means is required to transfer the generated heat away from the spindle and the parts within the spindle. If the parts within the spindle are operating at high temperatures then they are more prone to wear and eventually to failure. In particular any seals between the piston and the spindle and the ram and the spindle are susceptible to damage at higher temperatures. Hammers are generally operated in very dusty environments and it is critical to the prolonged operation of the hammer that there is no dust ingress into the spindle. As there are several ports in the spindle through which air can flow into and out of the spindle, cooling of the spindle using air flows can easily introduce dust into the spindle.
Therefore, cooling of the spindle is generally achieved by passive heat transfer from the metal spindle either via air pockets or directly to metal housing parts surrounding the spindle. However, the cooling achieved by such passive heat transfer is relatively limited.
The present invention aims to overcome the problems discussed above by providing a system which both reduces the vibration of the hammer housing and cools the spindle, without taking up much space within the hammer housing.
According to the present invention there is provided a hand held electrically powered hammer, comprising a housing within which is located:
a motor;
hollow spindle within which is located for reciprocation therein a piston and forwardly of the piston a ram;
a hammer drive arrangement which converts the rotary drive of the motor to a reciprocating drive to the piston;
a tool holder body located at the forward end of the spindle in which a tool or bit may be releasably mounted for limited reciprocation;
wherein the reciprocation of the piston reciprocatingly drives the ram via a closed air cushion such that repeated impacts from the ram are transmitted to a tool or bit mounted in the tool holder body, wherein the hammer additionally comprises:
a metal casing which encloses at least part of the spindle so as to form an air filled chamber between the spindle and the casing;
a damping mass, which is located within the chamber, which damping mass is connected to the hammer housing via at least one spring element so as to oscillate back and forth along the spindle to minimise the vibration of the hammer housing; and
at least one spacer element for positioning the damping mass with respect to the spindle and the metal casing so that a small gap is present between the mass and the spindle and a small gap is present between the mass and the casing such that oscillation of the damping mass within the chamber generates air turbulence within the chamber for facilitating heat transfer from the spindle to the metal casing.
The use of a damping mass oscillating within a chamber surrounding the spindle for reducing the vibration of the hammer housing is also used according to the present invention for generating air turbulence between the spindle and a metal casing part surrounding the spindle. When the damping mass moves forwardly along the spindle an overpressure is generated in front of the mass which causes air to flow rearwardly through the gaps between the mass and the spindle and the mass and the metal housing. When the damping mass moves rearwardly along the spindle an overpressure is generated rearwardly of the mass which causes air to flow forwardly through the gaps between the mass and the spindle and the mass and the metal housing. This air turbulence between the spindle and the metal casing can facilitate a three times increase in heat transfer away from the spindle as compared to passive heat transfer via an air pocket in which no turbulence occurs. According to the present invention the same components are used for the dual purpose of reducing the vibration transmitted to a user of the tool from the hammer housing and for cooling the spindle to improve the operation and lifetime of the hammer.
The hammer according to the present invention may comprise a beatpiece located for reciprocation within the spindle between the ram and a tool or bit mounted within the tool holder body for transferring impacts from the ram to a tool or bit mounted within the tool holder body. The incorporation of a beatpiece improves the sealing of the interior of the spindle from the tool holder body through which dust may enter.
For reducing any compensating vibrations due to the oscillation of the damping mass in a direction which is not parallel to the spindle, the metal casing and the the damping mass preferably encircle the spindle and the damping mass is preferably mounted so that it is concentric with the spindle. For a simple calibration of the mass and the spring or springs to compensate for vibrations in other parts of the hammer it is preferred that the damping mass comprises a single piece cylinder. Preferably, the mass is connected to the hammer housing via two springs one located forwardly of the mass between the mass and a forward housing part and the other located rearwardly of the mass between the mass and a rearward housing part. It is further preferred for a simple design in which the oscillating motion of the mass is easily controlled that the spring or each spring is a coil spring which encircles the spindle. Preferably, the mass is made of a relatively high density material such as steel or brass so that the mass does not take up too much space. For optimised vibration reduction in the hammer housing, the mass and the spring or springs are preferably arranged so that the mass oscillates back and forth along the spindle out of phase, preferably approximately 180xc2x0 out of phase, with the beat frequency of the other hammer parts.
The air turbulence in the chamber preferably includes air flows between the mass and the spindle and air flows between the mass and the metal casing.
The or each spacer element may be formed integrally with the damping mass. Alternatively, the or each spacer element may comprise a guide arrangement which is slideably mounted on the spindle. The damping mass may be mounted on such a guide arrangement and the guide arrangement may be shaped to form at least one channel between the damping mass and the spindle through which air can flow. Preferably, the at least one channel is formed between a radially inward facing part of the guide arrangement and the outer surface of the spindle. This increases the amount of air flow over the surface of the cylinder to aid cooling. However, the location of the channels between a radially inward facing part of the guide arrangement and the outer surface of the spindle will also reduce the surface area of contact between the guide arrangement and the spindle and so can reduce the friction generated between the guide arrangement and the spindle as the guide arrangement slides back and forth along the spindle, which again facilitates improved cooling of the spindle. In an especially preferred embodiment in which the damping mass and the magnesium casing encircle the spindle, the or each guide arrangement is a guide ring, and preferably two such guide rings are used, one located at either end (forward and rearward end) of the damping mass. Where the guide arrangement is one or more guide rings, the channels may be formed between ribs formed on the radially inward facing surface of the guide ring. The use of such ribs also reduces the surface area of engagement between the guide ring and the spindle which will reduce the friction generated as the guide ring slides along the spindle.
The hammer according to the present invention may additionally comprise a fan arrangement for generating an airflow and a labyrinth formed by parts of the hammer housing for directing the airflow over the outer surface of the metal casing. Having an airflow over the metal casing, which airflow may be a flow of dusty air from the environment of the hammer, facilitates heat transfer from the metal casing. By cooling the metal casing in this way the cooling of the spindle via the turbulent air in the chamber is further improved. The fan may be rotatingly driven by the motor to avoid a need for extra means on the hammer for powering the fan. Preferably, the fan generates an airflow which passes over the motor, through the fan and then through the labyrinth and over the metal casing before being exhausted from the hammer housing. Thus, the fan can perform the dual function of cooling the motor and cooling the metal casing to facilitate cooling of the spindle. The fan is preferably a radial fan.
The present invention is particularly suited for use in a heavy duty demolition hammer wherein the hammer drive arrangement comprises a crank arm arrangement. The more powerful hammers have a higher requirement for cooling of the spindle.
The hammer housing may comprise an inner metal housing arrangement in which the motor, hammer drive arrangement and at least part of the spindle are mounted and an outer plastic housing rigidly fixed to the inner metal housing which outer housing comprises a handle. In this case the metal casing surrounding the spindle may be rigidly fixed to a forward portion of the inner metal housing arrangement. Then the damping mass may connected to the hammer via a first forward spring which extends between the mass and a part of the metal casing and via a second rearward spring which extends between the mass and a part of the metal housing arrangement.
Preferably the air filled chamber between the spindle and the casing communicates with at least one other air space formed within the hammer, for example with the interior of the inner metal housing arrangement and/or with a space between the ram and the beatpiece. This is important if the chamber surrounds the vent holes in the spindle through which air must pass to vent the air cushion between the piston and the ram on entry into idle mode.